The present invention relates in general to a high emittance glass coating which is provided on surfaces of various kinds of structures for the purpose of increasing an emissivity of the structures, and a method of producing the high emittance glass coating. Also, the present invention is concerned with an improvement in a high emittance glass coating material which is used for forming the high emittance glass coating.
A heat insulator system serving for an aerospace or ultra-supersonic aviation, for example, is required to have an excellent heat resistance and a high emittance. Thus, a structural body used for such a purpose is provided on its surface with a glass coating having high emittance, such as a light-weight inorganic fibrous refractory which constitutes an outer wall of a space shuttle of the National Aeronautics and Space Administration (NASA). An example of the glass coating is disclosed in U.S. Pat. No. 4,093,771. This glass coating is constituted, for example, by a glass structure composed of a reaction cured glass (RCG) of a high silica borosilicate, and a high emittance pigment consisting of molybdenum disilicide (MoSi2) or a boron silicide compound such as silicon tetraboride (SiB4) or silicon hexaboride (SiB6), such that the high emittance pigment is dispersed in the glass structure. Thus, the glass structure is constituted by the reaction cured glass of the high silica borosilicate which has a high degree of heat resistance, and the high emittance pigment is dispersed in the glass structure, thereby providing the glass coating with a high heat resistance and a high emittance.
The above-described glass coating is produced, for example, as follow. That is, a predetermined amount of boron oxide is first mixed with a glass powder including a high silica glass. The mixture is fired and ground, for thereby producing a reaction cured glass powder. Next, the high emittance pigment is added to the thus produced reaction cured glass powder, for preparing a glass paste. The glass paste is applied to the surface of the light-weight refractory or other structural body. The applied glass paste is dried and then fired so that the glass structure constituting the above-described glass coating is formed from the glass powder. It is known that restraining oxidation of the high emittance pigment in the firing step is essential in the above production process where a non-oxide such as the above-described boron silicide compound is used as the high emittance pigment, in view of a fact that the boron silicide compound is decomposed into silicon oxide and boron oxide when the boron silicide is oxidized, whereby the pigment no longer provides the required optical properties such as high emissivity.
According to the technique disclosed in the above-identified U.S. Pat. No. 4,093,771, the decomposition of the high emittance pigment is restrained by rapidly heating the glass paste in the firing step. That is, the rapid heating of the glass paste leads to a rapid melting of the glass powder, and accordingly the high emittance pigment is rapidly covered by the molten glass powder, thereby restraining the oxidation of the high emittance pigment. However, even where the glass paste is thus rapidly heated, the high emittance pigment is gradually decomposed in the process of the firing operation, as shown in the schematic chart of the reaction process described in the above-identified patent. That is, the decomposition of the high emittance pigment is not satisfactorily restrained in the disclosed technique.
A further study by the present inventors with the purpose of producing a glass coating having higher emittance revealed that the reduction in the emissivity is due not only to the oxidation of the high emittance pigment, but also to the fact that the high emittance pigment is melt into the glass structure constituting the glass coating in the firing step. That is, the study of the present inventors revealed that such a reaction between the high emittance pigment and the glass structure is a cause for the reduction of the emissivity. Further, the reaction causes a change in the composition of the glass structure, resulting in a reduction in the heat resistance of the glass coating. As described above, the glass coating is provided on the surface of the refractory or other structural body for the purpose of increasing the emissivity of the structural body which is used at a high temperature. Namely, the glass coating is repeatedly or always exposed to a high temperature while the structural body is used. Accordingly, the glass coating suffers from the problematic reduction in the emissivity and the deterioration of the heat resistance due to the interface reaction between the glass structure and the high emittance pigment not only while the glass coating is produced but also while it is used, and moreover, independently of whether the pigment is of a non-oxide or an oxide.
The present invention was developed under the above-described background situation and has a first object of providing a high emittance glass coating material which is capable of suitably restraining the interface reaction between the glass structure and the high emittance pigment that is dispersed in the glass structure while the high emittance glass coating material is produced or while the high emittance glass coating is used. The present invention further has a second object of providing a high emittance glass coating, and a third object of providing a method of manufacturing the high emittance glass coating.
The above first object may be achieved by a first feature of the present invention, which provides a high emittance glass coating material which is applied to a surface of a specific structural body and fired on the surface, for providing on the surface a glass coating having a glass structure in which pigment particles having a predetermined degree of emissivity are dispersed, the high emittance glass coating material being characterized by including: (a) a pigment covering film having a predetermined thickness, which is provided to cover each of the pigment particles, and which includes silicon dioxide such that a content of the silicon dioxide in the pigment covering film is higher than a content of the silicon dioxide in each portion of the glass structure which is adjacent to a corresponding one of the pigment particles.
According to the first feature of the present invention, the high emittance glass coating material includes the pigment covering film having the predetermined thickness, which is provided to cover each of the pigment particles, and which includes the silicon dioxide such that the content of the silicon dioxide in the pigment covering film is higher than the content of the silicon dioxide in each portion of the glass structure which is adjacent to the corresponding one of the pigment particles. Thus, the glass coating which is obtained by firing the glass coating material applied to the surface of the structural body, is provided, at its interface between each pigment particle and the glass structure, with the pigment covering film which has a comparatively low reactivity with the pigment particle owing to its higher content of the silicon dioxide than that in the glass structure. The provision of the pigment covering film at the interface permits effective restraint of an interface reaction between the pigment particles and the glass structure while the glass coating material is fired or while the glass coating is used. That is, the interface reaction can be suitably restrained by increasing, at the interface between the pigment particle and the glass structure, the purity of the silicon dioxide which is chemically stable.
The above-indicated second object may be achieved by a second feature of the present invention, which provides a high emittance glass coating which is provided on a surface of a specific structural body and which has a glass structure in that pigment particles having a predetermined degree of emissivity are dispersed, the high emittance glass coating being characterized by including: (a) a pigment covering film having a predetermined thickness, which is provided to cover each of the pigment particles, and which includes silicon dioxide such that a content of the silicon dioxide in the pigment covering film is higher than a content of the silicon dioxide in each portion of the glass structure which is adjacent to a corresponding one of the pigment particles.
According to the second feature of the present invention, high emittance glass coating includes the pigment covering film having a predetermined thickness, which is provided to cover each of the pigment particles, and which includes silicon dioxide such that the content of the silicon dioxide in the pigment covering film is higher than the content of the silicon dioxide in each portion of the glass structure which is adjacent to the corresponding one of the pigment particles. Thus, the glass coating is provided, at its interface between each pigment particle and the glass structure, with the pigment covering film which has a comparatively low reactivity with the pigment particle owing to its higher content of the silicon dioxide than that in the glass structure. The provision of the pigment covering film at the interface permits effective restraint of an interface reaction between the pigment particles and the glass structure while the glass coating is used in the above-described first and second
In the above-described first and second inventions, (b) the above-described glass structure is preferably a borosilicate glass which includes the silicon dioxide as its principal component and also boric acid such that the content of the silicon dioxide in the above-described each portion of the glass structure is approximately 80 (wt %). Since the borosilicate glass is a glass having a high heat resistance, it is possible to obtain a glass coating of the structure which is suitable for use where a further higher heat resistance and a further higher emissivity are required. As the above-described borosilicate glass, for example, it is preferable to use a reaction cured glass which is obtained by firing a high purity silica glass whose content of the silicon dioxide is about 96(%) and a boron oxide which has been added to the high purity silica glass such that a content of the boron oxide in the sum of the high purity silica glass and the boron oxide is several (%), or alternatively a borosilicate glass whose content of the silicon dioxide is approximately 81(%). In the former reaction cured glass which is produced from the high purity silica glass particles, the high purity silica glass particles and the boron oxide added to the high purity silica glass are fired so that boron penetrates into each of the high purity silica glass particles whereby a layer of the borosilicate is formed on the surface, leading to a reduction in the content of the silicon dioxide at the surface. Thus, the content of the silicon dioxide in each portion of the glass structure which is adjacent to the corresponding pigment particle would be as low as about 80(%), whatever glass is used as the borosilicate glass, possibly causing an interface reaction which reduces the optical properties of the pigment particles, without the provision of the pigment covering film or pigment covering layer. From the point of view of the heat resistance, it is desirable that the silicon dioxide purity of the borosilicate glass be maximized. To this end, it is desirable that the borosilicate glass contain a minimum amount of impurities, in particular, sodium (Na), potassium (K) and other alkaline metals, magnesium (Mg), calcium (Ca) and other alkaline earth metals, iron (Fe), titanium (Ti), and lead (Pb), which tend to reduce the heat resistance of the borosilicate glass. The content of the impurities in the borosilicate glass is preferably 1 (wt %) less.
Further, the content of the silicon dioxide in the pigment covering film or pigment covering layer is preferably at least 85 (wt %). In this arrangement, the interface reaction between the pigment particles and the glass structure is further restrained owing to the sufficiently high content of the silicon dioxide. Also where the glass structure is constituted by the above-described borosilicate glass, for example, the content of the silicon dioxide in the pigment covering film or pigment covering layer is sufficiently higher than the content of the silicon dioxide in the portions of the glass structure which are adjacent to the respective pigment particles.
Further, (a-2) the content of the silicon dioxide in the pigment covering film or pigment covering layer is preferably at least 99 (wt %). In this arrangement, the interface reaction between the pigment particles and the glass structure is further assuredly restrained owing to the extremely high content of the silicon dioxide.
Further, (a-3) an average thickness of the pigment covering film or pigment covering layer is preferably about 0.5 (xcexcm). This average thickness of the pigment covering film or pigment covering layer is large enough to further assuredly restrain the interface reaction between the pigment particles and the glass structure, while at the same time this average thickness is small enough to avoid breakage of the pigment covering film or pigment covering layer due to a difference in coefficient of thermal expansion between the pigment covering film or pigment covering layer and the pigment particles during the formation of the pigment covering film or pigment covering layer, or to avoid a considerable influence on the thermal properties of the glass coating such as its softening point or thermal expansion coefficient. Thus, this average thickness avoids reduction in the optical properties of the pigment particles without particularly deteriorating the function of the glass coating.
Further, (a-4) a thickness of the pigment covering film or pigment covering layer preferably ranges from about 0.1(xcexcm) to several(xcexcm). This thickness of the pigment covering film or pigment covering layer is large enough to further assuredly restrain the interface reaction between the pigment particles and the glass structure, while this thickness is small enough to avoid breakage of the pigment covering film or pigment covering layer due to a difference in coefficient of thermal expansion between the pigment covering film or pigment covering layer and the pigment particles during the formation of the pigment covering film or pigment covering layer, or to avoid a considerable influence on the thermal properties of the glass coating such as its softening point or thermal expansion coefficient. Thus, this thickness range avoids reduction in the optical properties of the pigment particles without particularly deteriorating the function of the glass coating.
Further, (c) each of the above-described pigment particles is preferably constituted by at least one of boron silicide such as silicon tetraboride or silicon hexaboride, molybdenum disilicide, silicon carbide, iron oxide, silicon nitride, and chromium oxide, each of which has a sufficiently high emissivity, thereby making it possible to form a high emittance glass coating having a high emissivity. It is further preferable that the pigment particle be the boron silicide. The boron silicide is further preferably used as the pigment particle since the boron silicide has an extremely high emissivity. The boron silicide has also a high reactivity with the glass structure since the boron silicide is not an oxide, so that the provision of the pigment covering film or pigment covering layer is further considerably effective to this arrangement in which the pigment particle is constituted by the boron silicide. It is still further preferable that the pigment particle be the silicon tetraboride, thereby obtaining a glass coating which maintains its high emissivity at a further higher temperature, owing to the fact that the optical properties of the silicon tetraboride are less likely to be affected at a high temperature, than those of other boron silicide.
Further, (c-1) each of the above-described pigment particles is a silicon tetraboride in the form of particles whose average diameter is approximately 2 (xcexcm). According to this arrangement, it is possible to sufficiently disperse the pigment particles in the glass structure, and also sufficiently increase the emissivity of the glass coating.
Further, (c-2) each of the above-described pigment particles is a silicon tetraboride in the form of particles whose diameters range from 1 to 10 (xcexcm). According to this arrangement, it is possible to sufficiently disperse the pigment particles in the glass structure, and also sufficiently increase the emissivity of the glass coating.
The above-indicated third object may also be achieved by the present third invention of the present invention, which provides a method of manufacturing a high emittance glass coating which has a glass structure in that pigment particles having a predetermined degree of emissivity are dispersed, and which is provided on a surface of a specific structural body, the method including: (d) a paste preparation step of preparing a paste which includes the pigment particles and a specific glass powder (e) a paste coating step of applying the paste to the surface of the specific structural body, and (f) a heat treatment step of forming the glass structure from the glass powder by heating the paste which has been applied to the surface, the method being characterized by including (g) a pigment-particle covering step of providing on surface of each of the pigment particles a pigment covering film having a predetermined thickness, the pigment covering film including silicon dioxide such that a content of the silicon dioxide in the pigment covering film is higher than a content of the silicon dioxide in each portion of the glass structure which is adjacent to a corresponding one of the pigment particles, the pigment-particle covering step being implemented prior to the paste preparation step.
According to the present method of manufacturing the high emittance glass coating, the pigment-particle covering step is implemented prior to the paste preparation step, for providing on the surface of each of the pigment particles the pigment covering film which has a predetermined thickness, and which includes silicon dioxide such that the content of the silicon dioxide in the pigment covering film is higher than the content of the silicon dioxide in each portion of the glass structure adjacent to the corresponding one of the pigment particles. Therefore, the paste prepared in the paste preparation step includes the glass powder and the pigment particles each of which is provided at its surface with the pigment covering film having a low reactivity with the glass structure owing to the high content of the silicon dioxide in the pigment covering film. The presence of the pigment covering film is effective to restrain the interface reaction between the glass structure and the pigment particles while the prepared paste is subjected to the heat treatment in the heat treatment step. Similarly, also after the high emittance glass coating has been produced, namely, also while the high emittance glass coating is used, the interface reaction is restrained owing to the presence of the pigment covering film.
In the above-described third feature of the present invention, (g) the above-described pigment-particle covering step preferably includes: (g-1) an inorganic-high-molecular-film forming step of forming an inorganic high molecular film on the surface of the each of the pigment particles, the inorganic high molecular film being constituted by an inorganic high molecule which includes silicon; and (g-2) a heating and forming step of heating the each of the pigment particles having the inorganic high molecular film formed thereon, at a predetermined temperature in an oxidizing atmosphere, so that the pigment covering film having the content of the silicon dioxide is formed from the inorganic high molecular film. According to the present method, the inorganic high molecular film including the silicon is formed on the surface of each of the pigment particles in the inorganic-high-molecular-film forming step, and each of the pigment particles is then heated in the oxidizing atmosphere for forming the pigment covering film from the inorganic high molecular film in the heating and forming step. Thus, the pigment covering film takes the form of the inorganic high molecule to be formed on the surface of the pigment particles, thereby making it possible to preferably form the pigment covering film with a small and constant thickness. Further, since the inorganic high molecule includes the silicon, the formed pigment covering film has a predetermined content of the silicon dioxide as a result of oxidation of the silicon included in the inorganic high molecule by heating the pigment particles in the oxidizing atmosphere. The pigment covering film having the predetermined thickness and the predetermined content of the silicon dioxide can be thus preferably formed.
Further, (g-1) the inorganic-high-molecular-film forming step includes: (g-1-1) a pigment-particle dispersing step of dispersing the pigment particles in a liquid including the inorganic high molecule, for preparing a dispersion liquid; and (g-1-2) a spray-drying step of spray-drying the dispersion liquid, for forming the inorganic high molecular film on the surface of the each of the pigment particles. According to the present method, the pigment particles are dispersed in the liquid including the inorganic high molecule for preparing the dispersion liquid in the pigment-particle dispersing step, and the dispersion liquid including the inorganic high molecule and the pigment particles is spray-dried for forming the inorganic high molecular film on the surface of each of the pigment particles in the spray-drying step. The liquid including the inorganic high molecule which covers the pigment particle is rapidly spray-dried, for thereby forming the inorganic high molecular film with its thickness made further small and constant.
Further, (g-1-3) a compound substantially constituted by hydrogen (H), nitrogen (N) and silicon (Si) is preferably used as the inorganic high molecule in the inorganic-high-molecular-film forming step. According to the present method, the silicon and the oxygen are bonded to each other as a result of the firing of the compound in the oxidizing atmosphere, thereby forming the silicon dioxide so that the inorganic high molecular film is formed on the surface of each pigment particle, while the hydrogen and the nitrogen are bonded to each other as a result of the firing of the compound in the oxidizing atmosphere, thereby forming ammonia (NH3) or gaseous hydrogen (H2) in addition to the ammonia, each of which is extinguished immediately after the formation. Thus, the formed inorganic high molecular film and also the pigment covering film constituted by the inorganic high molecular film are permitted to include an extremely high content of the silicon dioxide, whereby the interface reaction between the pigment particles and the glass structure is further restrained. As the above-described compound, perhydropolysilazane is preferably used, and the compound may include a small amount of oxygen (O) or carbon (C) in addition to the above-described elements.
Further, (d-1) the pigment particles and the glass powder are preferably dispersed together with an organic binder in an organic solvent in the above-described paste preparation step. According to this method, the paste is prepared by dispersing the pigment particles and the glass powder in the organic solvent, thereby making it possible to form a further uniform glass coating. Further, the inclusion of the organic binder in the paste permits the paste to have a suitable thickness when the paste is applied to the surface of the above-described structural body. The amounts of the organic binder and organic solvent to be used are determined by taking account of the viscosity of the paste.
Further, (e-1) the paste is preferably sprayed on the above-described surface in the above-described paste application step. According to this method, it is possible to easily form a coating on the surface of the structural body with its thickness substantially constant. Further, (f-1) the paste is heated in a non-oxidizing atmosphere in the above-described heat treatment step. According to this method, the oxidation of the pigment particles is further restrained during the heat treatment in the absence of oxygen in the firing atmosphere. This further reduces the necessity of rapidly heating and cooling the paste for the purpose of preventing the oxidation, whereby the glass coating is formed by raising and lowering the temperature in accordance with a desired temperature curve which minimizes the distortion of the structural body, and accordingly making it possible to produce the structural body provided with the glass coating, with a high geometric accuracy.
Further, (d-2) a borosilicate glass which includes the silicon dioxide as its principal component and also includes boric acid is used as the above-described glass powder in the above-described paste preparation step. According to this method, since the borosilicate glass is a glass having a high heat resistance, it is possible to obtain a glass coating of the structure which is suitable for use where a further higher heat resistance and a further higher emissivity are required. As the borosilicate glass, the above-described reaction cured glass or borosilicate glass or other glass is preferably used. The borosilicate glass preferably has a maximum content of the silicon dioxide purity and a minimum content of impurities, such as sodium, potassium and other alkaline metals, magnesium, calcium and other alkaline earth metals, iron, titanium, and lead, which tend to reduce the heat resistance of the borosilicate glass. The content of the impurities in the borosilicate glass is preferably at least 1 (wt %).